For automatic transmissions for motor vehicles, for example, from DE 198 58 541 A1, the transmission ratio stages are adjusted by shifting elements, which are frictional-locking couplings or brakes. In the disclosed automatic transmission, couplings that can connect two rotatable elements to each other in a torque-proof manner, and brakes, which connect in a torque-proof manner a rotatable element to a fixed element or to a transmission housing, are formed as multi-disk shifting elements.
In principle, the shifting elements of an automatic transmission can be formed both as frictional-locking multi-disk shifting elements and as positive-locking shifting elements, such as, dog clutches. In automatic transmissions, at least one of the shifting elements is formed to be frictional-locking, in order to carry out power shifts; that is, a change of transmission ratio without interruptions in the pulling force.
With known automatic transmissions, the actuation (that is, the closing of the shifting elements for the transfer of torque) is effected hydraulically; that is, through actuators in the form of piston/cylinder units, which are subjected to pressurized oil. An actuating pressure chamber is formed from the piston and the cylinder, and the shifting element is actuated by pressure oil under an actuating pressure. The pressure oil is conveyed by a motor-driven pump as a pressure source and, during the entire operating period of the automatic transmission, must be held at a pressure level that generates a pressing force of the multi-disks in the shifting element that is sufficient for the transfer of torque. The energy for generating the pressure by the hydraulic pump must be applied by the motor of the motor vehicle, such that the generation of the hydraulic power increases the fuel consumption and therefore CO2 emissions, or reduces the energy available for driving the vehicle due to the power losses. This also reduces the transmission efficiency.
In addition, leakage losses are generated at sealing points such as, for example, pressure oil feeds from the transmission housing through so-called “rotary oil feeds,” which are sealed by gap seals (for example, sliding bearings and/or rectangular rings) in the rotating transmission shaft. These require a continuous tracking of the oil pressure in the actuator with a closed shifting element or a replenishment of the leakage quantity, in order to keep the shifting element closed.
In order to make the pressure in the actuator independent of the pressure of the pump, and to minimize the leakage losses, the piston chamber can be blocked by various locking mechanisms, such that the pressure remains in the piston chamber and no further oil has to be conveyed. Only during the shifting process is the valve opened and then filled with the corresponding pressure. A hydraulic control device is known from DE 102 05 411 A1 of the applicant, with which, in the event of a desired transfer of torque, the multi-disk shifting elements that can be closed by pressurization are locked outside the gearshifts by a blocking device, in which a shut-off valve is closed in the feed area of the shifting element. As a result, the pressure in the actuating cylinder, acting as an actuator, and thus the pressing force between the multi-disks, is maintained, without a hydraulic pressure having to be generated by the transmission pump in the magnitude of the actuating pressure of the shifting element.
Depending on the application, shut-off valves can be formed in such a manner that they are “normally closed” or “normally open” with a system pressure at the level of the ambient pressure.
Thus, the pressure to be generated by the transmission pump can be lowered with respect to the actuating pressure enclosed in the shifting element. However, although theoretically possible, the pump is not completely switched off or depressurized since, even with closed and locked shifting elements, the need for oil is maintained at a low pressure level in the transmission, for example for cooling and lubrication or for pre-filling the shifting element to prepare a shifting operation. The power consumption of the transmission oil pump, which is calculated as the product of the delivered volume flow and the generated pressure difference, is significantly reduced by the possible reduction in pressure. With a lower power consumption of the transmission pump, the overall efficiency of the transmission increases, since less engine power has to be branched off as idle power for the hydraulics and is available for the vehicle drive.
However, if, in the closed state of the shut-off valve and thus of the shifting element, the torque of the transmission to be transferred is increased such that the enclosed actuating pressure would have to be increased, the shut-off valve must be opened, which results in a decline in pressure in the shifting element pressure chamber and thus, disadvantageously, in a brief opening of the shifting element or an interruption in the pulling force. Thus, an adjustment of the actuating pressure acting on the frictional-locking shifting element is also not possible in the case of the theoretically possible occurrence of undesired leakages, which can lead to a decrease in the actuating pressure.